There are various types and different sizes of wind energy converters which can be found in operation in several regions of the world. Wind energy converters can be classified according to the relative position of the axis of rotation of the wind rotor with respect to the wind direction into two main categories: horizontal axis wind turbines (HAWT) and vertical axis wind turbines (VAWT) (Erich H, 2006; David S, 2009). Most commercial wind turbines today belong to the horizontal-axis type, in which the rotating axis of blades is parallel to the wind stream. The HAWT category includes the modern high tip speed ratios wind turbines with two and three blades. The advantages of a HAWT include high generating capacity, improved efficiency, variable pitch blade capability, and tall tower base structure to capture large amounts of wind energy. HAWTs with two or three blades are the most common (Jha A R, 2011).
In the other VAWTs category, the rotor axis is in cross direction with respect to the wind stream. VAWTs include Savonius rotor, Darrieus wind turbine, Giromill wind turbine, and others. VAWTs have been used to generate mechanical and electrical energy at a range of scales, from small-scale domestic applications through to large-scale electricity production for utilities. In recent years there has been a resurgence in vertical axis wind turbine development for both urban-scale or off-grid applications as well as off-shore as alternatives to horizontal axis wind turbines (Kjellin et al 2011; Howell et al 2010; Greenblatt et al 2012; Islam M 2008; Ferreira et al 2009; Akwa J V 2012). A significant advantage of most designs of the vertical-axis wind turbines is that they are insensitive to wind direction and can accept wind from any direction and thus no yaw control is needed (Greenblatt et al 2012). Since the wind generator, gearbox, and other main turbine components can be set up on the ground, it greatly simplifies the wind tower design and construction, and consequently reduces the turbine cost. VAWTs in principle can attain a maximum coefficients of performance, Cp,max, that are comparable to those for HAWTs and they have several potentially significant advantages over the HAWTs. However, some types of the vertical-axis wind turbines, like Darrius and Cochrane turbines, must use an external energy source to rotate the blades during initialization. Because the axis of the wind turbine is supported only on one end at the ground, its maximum practical height is thus limited. Due to the lower wind power efficiency, vertical-axis wind turbines today make up only a small percentage of wind turbines (Wei T 2010).
According to the aerodynamic function of the rotor, wind energy converters can also be classified in to two types. The rotor's aerodynamic function is characterized by the fact of whether the wind energy converter captures its power exclusively from the aerodynamic drag of the air stream acting on rotor surfaces, or whether it is able to utilize the aerodynamic lift created by the flow against suitably shaped surfaces. Accordingly, there are so-called “drag-type rotors” and “lift-type rotors” (Erich H, 2009).
The “Savonius rotor”, which can be found as ventilator on railroad carriages or delivery vans, and the cup anemometer used to measure wind velocity, are well-known examples of drag-type rotors with a vertical axis of rotation (Kacprzak et al 2013; Manwell et al 2010). The Darrieus rotor which is built with two or three rotor blades, and more efficient than Savonius rotor, utilizes aerodynamic lift. Another version of the Darrieus rotor is the so called H-Darrieus rotor. Instead of curved rotor blades, straight blades connected to the rotor shaft by struts are used (Eric H, 2006). FIG. 1 displays power coefficients as functions of blade tip speed to wind speed ratio for propeller type windmills, turbines using high-speed two-bladed rotors, turbines with multi-bladed rotors, turbines with Savonius rotor configurations, and Dutch type turbines.
There are still issues with the energy being wasted during the rotation and achieving more efficiency. There is a need for a refined design to mitigate these factors and be cost efficient.